With the rapid increase in wireless communication, there is an increasing need for mobile devices, such as portable computers, laptops, palmtops, personal digital assistants and similar devices (hereinafter collectively referred to as mobile computing devices), to be able to communicate wirelessly with a variety of services. At the present time, a range of wireless services are in common use, for example wireless LANs, GSM, GPS and similar. These encompass communication services such as GSM or Bluetooth as well as geographical positioning systems such as GPS.
These different wireless communication systems, each with corresponding different operating frequencies, will continue to be used in the foreseeable future. With the convergence of device functionality, for example, a mobile phone integrated with a PDA, it is envisaged that such a single device would be capable of handling communications in respect of a variety of services.
The frequencies allocated to the different services reflect a number of factors including statutory allocation schemes, technical suitability to a specific type of task or historical precedent. It is envisaged that these plural communication systems will continue in existence given the advantages they offer in their own particular domains as well as for legacy reasons.
For devices requiring multiple-access, that is, the ability to simultaneously receive and transmit on different frequency bands, usually using different communication standards, it is necessary to provide an antenna assembly which provides such functionality.
Attempts have been made to design antenna assemblies for mobile computing devices which are able to operate at two different wireless communication frequencies. For example, M. Ali et al, in an article entitled “Dual-Frequency Strip-Sleeve Monopole for Laptop Computers”, IEEE Transactions on Antennas and Propagation, Vol. 47, No. 2, February 1999, pp. 317–323, describes a monopole antenna design which can operate at two frequencies, namely between 0.824–0.894 GHz for the advanced mobile phone systems (AMPS) band and between 1.85–1.99 GHz for the personal communication systems (PCS) band. Ali et al describes the satisfactory operation of a strip-sleeve monopole antenna within these two frequency bands, including the possibility of omitting one of the two sleeves. A strip-sleeve antenna in this context corresponds to a single monopole with two parasitic antennas arranged on either side of the primary monopole, thus, when viewed from the side, constituting a sleeve arrangement. A three-dimensional analogue is a coaxial sleeve antenna. The system described by Ali et al is however limited to dual frequency applications over a fairly narrow range of frequencies.
Although several antenna solutions already exist in the market for the different wireless communication standards described below, they are generally individually expensive, particularly if it is desired to provide a plurality of antennae to be able to scan all of the communication bands which are accessible. These solutions are therefore not practicable and may further suffer from the drawback that when located in the same device, each may interfere with the others operation.